The period covering Christmas and the new year is notorious for family breakdown. Fractious partners look back at the year that is passing, look ahead to the one coming - and decide to strike out on their own, if that is financially possible.  This logic can also be applied to the troubled cohabitation that is the Coalition. Nick Clegg is in the position of a cashstrapped partner who cannot afford to leave the family home: the polls suggest that an election now could halve the number of Liberal Democrat MPs. However, David Cameron has money in the bank, at least as far as those same surveys are concerned. The last poll of 2011 found his party ahead of Ed Miliband's. The penultimate one saw its ratings touch 40 per cent - a four-point increase on its general election total.  Conservative members and activists have been encouraged by these findings: one survey found that over half believe that a snap election would produce a Tory majority. Some of the more excitable spirits among the grassroots continue to enthuse about a poll. and there is no reason to suppose that the recent surge in the party's ratings won't be repeated, especially if the Prime Minister follows through his EU summit veto. So, given the agonies of the Liberal Democrats and the weakness of Ed Miliband, why won't Cameron call the election which would give the country the Tory government it needs?  The question should meet a brutal threepart answer. First, this happy outcome simply wouldn't happen. Under the present constituency boundaries, the Tories must be roughly eight points ahead of Labour to win a majority. Even the sunniest poll for the party shows them nowhere near achieving and sustaining a lead of this size.  Second, Cameron can't call an election. Supporters of an early contest have forgotten about the Fixed Term Parliaments act. Conceived during the Coalition negotiations and effected this year, the measure ended the practice whereby the Prime Minister can simply go to the Palace and ask for an election - which was precisely why the Liberal Democrats insisted on it.  Third, such a gambit would be dubious even had the act not been passed. Voters may not love the Government. But they detest unnecessary elections. and despite record back-bench rebellion rates, the Government has an emphatic and workable majority in the Commons of more than 80. It has given itself the task of eliminating the structural deficit. Until or unless the deficit is under control, it would be a dereliction of duty to break up the Coalition.  In short, Conservative and Liberal Democrat MPs may be restive (and the parties' activists even more so) but the Cameron and Clegg relationship, however tense and strained, will remain in place, based on mutual need and convenience. There is, however, an alternative both to wistful dreams of separation and the chafing reality of their present life together.  Last spring's referendum rejection of aV ended the rose garden politics of the Coalition's opening year. The ease that the two leaders displayed in their first (and only) joint press conference was succeeded by edginess. Under pressure from his party, Clegg began to define the Liberal Democrats not by what they were achieving in government, but by what they were preventing the Conservatives from doing.  Thus they came out against andrew Lansley's health reforms. against any reduction in the 50p tax rate. against the Beecroft report's original plans to boost small businesses. against plans for a Bill of Rights. and against the support for marriage in the tax system championed by Iain Duncan Smith a